JavaScript Function Definitions
Function Declarations
A function declaration uses the function keyword and a function name.
Function declarations are loaded before the code runs.
Example
function add(a, b) {
return a + b;
}
add(4, 5);
You can call a function declaration before or after it is written in the code.
Function Expressions
A function expression stores a function inside a variable.
The function can be anonymous (without a name).
Example
const add = function(a, b) {
return a + b;
};
add(4, 5);
Function expressions are executed only when the code reaches them.
Declarations vs Expressions
The difference is when the function becomes available.
Example
add1(4, 5);
function add1(a, b) {
return a + b;
}
Example
add2(4, 5); // Error
const add2 = function(a, b) {
return a + b;
};
In the first example, the function works before it is defined.
In the second example, the function does not exist until the code reaches it.
Why This Happens (Hoisting)
Function declarations are hoisted to the top of their scope.
Function expressions are not hoisted in the same way.
This means:
- Function declarations can be called before they appear
- Function expressions cannot be called before they are defined
Functions Stored in Variables
A function stored in a variable can be used like any other value.
Example
const myFunction = function() {
return "Hello";
};
let text = myFunction();
Function Declarations
Earlier in this tutorial, you learned that functions are declared with the following syntax:
function functionName(parameters) {
// code to be executed
}
Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are invoked (called upon).
Note
Semicolons are used to separate executable JavaScript statements.
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.
Function Expressions
After a function expression has been stored in a variable, the variable can be used as a function:
The Function() Constructor
As you have seen in the previous examples, JavaScript functions are defined
with the function keyword.
Functions can also be defined with a built-in JavaScript function constructor called
Function().
Example
const myFunction = new Function("a", "b", "return a * b");
let x = myFunction(4, 3);
Try it Yourself »
You actually don't have to use the function constructor. The example above is the same as writing:
Example
const myFunction = function (a, b) {return a * b};
let x = myFunction(4, 3);
Try it Yourself »
Most of the time, you can avoid using the new keyword in JavaScript.
Function Hoisting
Earlier in this tutorial, you learned about "hoisting" (JavaScript Hoisting).
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.
Hoisting applies to variable declarations and to function declarations.
Because of this, JavaScript functions can be called before they are declared:
myFunction(5);
function myFunction(y) {
return y * y;
}
Functions defined using an expression are not hoisted.
Functions Can Be Used as Values
JavaScript functions can be used as values:
JavaScript functions can be used in expressions:
Functions are Objects
The typeof operator in JavaScript returns "function" for
functions.
But, JavaScript functions can best be described as objects.
JavaScript functions have both properties and methods.
The arguments.length property returns the number of arguments received when
the function was invoked:
The toString() method returns the function as a string:
Example
function myFunction(a, b) {
return a * b;
}
let text = myFunction.toString();
Try it Yourself »
A function defined as the property of an object, is called a method to the object.
A function designed to create new objects, is called an object constructor.
